In the two-phase interfacial process for the production of polycarbonates, a better conversion of the raw materials and effective phase separation can be obtained by conducting the reaction in such a way that very few, if any, unwanted secondary reactions take place and the emulsion can readily be separated. There has been no shortage of efforts in this direction.
Thus, DOS 2,305,144 describes a process for the continuous production of polycarbonates, in which the two reactive phases are combined in the presence of amines in a mixing zone under essentially oil-in-water emulsion conditions and the phosgenation step takes place in a reaction zone after mixing. Specially designed flow arrangements are intended to ensure that the volume-time yield of the reaction is increased. The disadvantage lies in the large quantity of aqueous phase which promotes the secondary phosgene reactions.
According to DOS 2,353,939, the properties of a polycarbonate produced by the two-phase interfacial method are said to be improved by controlling the reaction through adjustment of the pH value. The disadvantage lies in the excess phosgene used, in addition to which the process is not continuous.
According to the teaching of EP 0,282,546, chloroformyl-terminated condensates are said to be obtained with high phosgene yields by a version of the two-phase interfacial process in which a stable diphenol/water/sodium hydroxide suspension and phosgene are continuously introduced at the same time into an organic phase and the reaction product is subsequently isolated. pH values of 2 to 5 are maintained during the reaction. Disadvantages arise out of the technical problems involved in dosing the suspension and the low pH value which considerably increases the phosgenation time. No measures for a polycondensation reaction are described.
According to the EP 0,263,432, chloroformyl-terminated condensates or polycarbonates can be produced from aqueous diphenolate solution and organic solution by introducing phosgene into a heterogeneous mixture at pH values of 8 to 11, at temperatures of 15.degree. to 50.degree. C. and with a phosgene excess of at least 10 mol- % and continuing the phosgenation reaction with simultaneous introduction of alkali metal or alkaline earth metal hydroxides. Preferred phase ratios are 0.4-1 to 1 water-to-oil ratios, water being subsequently added.
According to DOS 2,725,967, it is favorable to the conversion of phosgene of a continuous process if the aqueous phase and the organic phase containing dissolved phosgene are initially combined in a tube and subsequently introduced into a reactor of the tank type. The residence time in this tube should be between 0.5 and 15 seconds. The phosgene excess of the reaction is at least 10 mol- %. The disadvantage lies in the fact that the phosgene excess is still extremely high. In addition, the process is attended by the disadvantage that the phosgenation reaction takes place at unfavorable phase ratios (oil-to-water 0.2 to 1) so that phase separation on completion of the reaction is definitely possible.
According to EP 0,306,838-A2, the phosgenation reaction is monitored in situ using an automatic chloride detector. Carrying out the process in which way suppresses variations in the chemism of the reaction and is said distinctly to improve the technical properties of the polycarbonates. The concept on which the process is based lies in the recycling of unreacted diphenolate to the process. However, the process is attended by the disadvantage of secondary phosgene reactions which even take place during recycling.
According to EP-0,339,502-A2, the secondary phosgene reactions are said to be increased by the presence of a high initial sodium hydroxide concentration. According to this patent, therefore, the diphenol/sodium hydroxide/water solution is combined with the organic phases in an alkali/hydroxy ratio of less than 2:1 (less than equivalent quantity of alkali metal hydroxide), oligomers having a molecular weight of 300 to 3,000 g/mol being formed in this first reaction stage. The water-to-oil phase ratios are greater than 1. In addition, the secondary phosgene reactions are still extremely favorable.
According to EP-0,304,691-A2, a fine emulsion produced by intensive mixing is favorable to the reaction in the two-phase interfacial process, albeit with a very large excess of phosgene (20 to 100 mol- % excess). The large quantity of phosgene promotes good phase separation despite intensive mixing of the emulsion at the beginning of the reaction. However, the conversion of phosgene to polycarbonate is extremely unfavorable.
German patent application P 4 118 232.4 (Le A 28 238) describes a special process for avoiding secondary phosgene reactions in which the sodium hydroxide solution is added in a special form.
To improve the volume-time yield of existing reactors, it was logical to increase the concentrations of the reactants. However, their solubility in the carrier medium (organic solvent or water) is only limited, so that higher temperatures have to be applied. The resulting faster reactions also lead to more secondary reactions which inter alia reduce the phosgene yield.